The invention relates generally to integrated circuits (ICs), and more particularly to an improved isolation structure and a method for forming the same. For example, the isolation structure reduces xe2x80x9chumpxe2x80x9d effects in transistors that are adjacent to the structure, particularly in transistors having a feature size of 0.25 microns (xcexcm) or less.
To meet the industry demand for ICs that pack greater functionality into the same or a smaller die area, IC manufacturers continue to research and develop processes that allow integrated devices such as transistors to have smaller geometries. For example, a few years ago, many IC manufacturers used a 4 xcexcm process, which can form devices having a feature size (e.g., the width of a transistor gate) as small as 4 xcexcm. But today, 1 xcexcm processes are common, and 0.25 xcexcm, 0.18 xcexcm and 0.1 xcexcm processes are under development. These smaller-geometry processes allow the formation of integrated devices having smaller geometries. Consequently, such processes allow more devicesxe2x80x94and thus more functionalityxe2x80x94on a given die area than larger-geometry processes do.
Unfortunately, merely scaling down the dimensions of an integrated device to take advantage of a smaller-geometry process may render the device inoperable. For example, due to known short-channel effects, a transistor having a gate width of 4 xcexcm may operate improperly if it is scaled down to have a gate width of 1 xcexcm.
FIG. 1 is a cross-sectional view of a conventional silicon-trench isolation (STI) structure 10, which is part of an IC 11 such as a memory circuit. The IC 11 includes transistors 12a and 12b, which are disposed in a substrate 13 having a surface 14 and corners 16a and 16b. The transistors 12a and 12b include respective body regions 18a and 18b, which are disposed in the substrate 13 and which electrically invert during transistor operation to form respective channel regions. Gate insulators 20a and 20b are respectively disposed on the body regions 16a and 16b. A conductor 22, such as a word line, extends over the isolation structure 10 and the gate insulators 20a and 20b and acts as a gate electrode for both the transistors 12a and 12b. 
Unfortunately, the isolation structure 10 may cause the transistors 12a and 12b to operate improperly. The isolation structure 10 includes an isolation trench 24 disposed in the substrate 13. The trench 24 is filled with an insulator 26 having side walls that often taper inwardly as they extend from the trench 24 above the surface 14 of the substrate 13. This narrowing forms gaps 28a and 28b, which allow the gate conductor 20 to closely overlap the corners 16a and 16b, respectively. During operation of the transistors 12a and 12b, this overlap often causes undesirable fringe, i.e., xe2x80x9chump,xe2x80x9d effects in the respective regions of the transistors"" electric fields near the corners 16a and 16b. If the transistors 12a and 12b have relatively large feature sizes, then these hump effects typically have only a negligible affect on transistor operation. But if the transistors 12a and 12b have relatively small feature sizes, particularly feature sizes of 0.25 xcexcm or less, then these hump effects may severely degrade the transistor operation, and may even render the transistors 12a and 12b unusable. Furthermore, even if the side walls of the insulator 26 are straight outside of the trench 24, the conductor 22 may still be close enough to the corners 16a and 16b to cause significant hump effects.
In one aspect of the invention, an IC isolation structure includes a recess disposed in a conductive layer having a surface portion. The recess has a side wall adjacent to the surface portion, and the isolation structure also includes an insulator disposed in the recess and overlapping the surface portion.
Thus, if a transistor is disposed in the conductive layer adjacent to the recess side wall, the overlapping portion of the insulator increases the distance between the recess corner and the gate electrode. This increased distance reduces hump effects to tolerable levels.